Chemistry · Amines · NEET
HNO2 is a weak, unstable acid. You cannot store it in a bottle. It is made fresh in the flask by mixing sodium nitrite (NaNO2) with a cold mineral acid like HCl: NaNO2 + HCl -> HNO2 + NaCl. This is why NEET writes the reagent as 'NaNO2/HCl' or 'NaNO2 + dil. HCl at 273-278 K'. They all mean the same thing: nitrous acid formed in place.
A primary aliphatic amine (R-NH2) first forms an aliphatic diazonium salt (R-N2+). But this ion has no benzene ring to spread out and hold the positive charge, so it is very unstable. It breaks apart instantly, throwing out nitrogen gas (N2) and leaving a carbocation. Water then attacks to give an alcohol. Net result: R-NH2 + HNO2 -> R-OH + N2 (gas) + H2O. You see bubbles of N2. This is why aliphatic amines do NOT give a usable diazonium salt.
In aniline, the -N2+ group is attached to a benzene ring. The ring lone pairs and pi electrons spread out (delocalise) the positive charge by resonance, so the salt is stabilised. But this stability only holds at low temperature, 273-278 K (0-5°C). If you warm it above about 278 K, it decomposes: the diazonium group is replaced by -OH and N2 escapes, giving phenol. So you must keep the reaction in an ice bath.
No. Only PRIMARY amines give diazonium chemistry. A secondary amine (R2NH) reacts with HNO2 to give a yellow oily N-nitrosamine (R2N-N=O). A tertiary aliphatic amine just forms a soluble nitrite salt (no useful product). A tertiary aromatic amine like N,N-dimethylaniline gives a green C-nitroso ring product (p-nitroso). So the reaction with HNO2 can help tell the three classes apart, but the clean N2-gas/alcohol test is only for primary amines.
Diazotisation means making the diazonium salt (-N2+) from a primary amine using NaNO2/HCl at low temperature. Deamination means removing the nitrogen: for an aliphatic amine this happens on its own (N2 leaves, alcohol forms), and for an aromatic diazonium salt you can force it with warm water (gives phenol) or with H3PO2 (gives just the C-H, the amino group is fully removed). NEET loves asking which reagent removes the diazonium group.
Because temperature decides the product. At 273-278 K (0-5°C) aniline gives the stable benzenediazonium chloride you can use for further reactions (Sandmeyer, azo coupling). Above this range the same salt decomposes to phenol. So the number 273-278 K is a signal in a question that diazotisation is happening. If a statement says the aromatic salt is stable 'above 300 K', that is FALSE (this is a common NEET trap).
Statement I: Primary aliphatic amines react with HNO2 to give unstable diazonium salts. Statement II: Primary aromatic amines react with HNO2 to form diazonium salts which are stable even above 300 K. Choose the most appropriate answer.
The major product Z formed in the following sequence of reactions is: C2H6 ->[Cl2/UV] X ->[NH3] Y ->[(i) NaNO2/HCl (ii) H2O] Z
A nitrogen-containing aromatic compound A reacts with Sn/HCl, followed by HNO2, to give an unstable compound B. B, on treatment with phenol, forms a coloured compound C with molecular formula C12H10N2O. The structure of compound A is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An alcohol plus nitrogen gas. For example ethylamine gives ethanol: C2H5NH2 + HNO2 -> C2H5OH + N2 + H2O. The escaping N2 gas (bubbles) is the tell-tale sign of a primary aliphatic amine.
Benzenediazonium chloride, C6H5N2+Cl-, when done cold at 273-278 K with NaNO2/HCl. This salt is useful for making many aromatic compounds through Sandmeyer, Gattermann and azo-coupling reactions.
Because the benzenediazonium salt is only stable while cold (273-278 K). If the mixture warms above about 278 K, the salt decomposes to phenol and N2 gas, and your product is lost.
Yes, partly. A primary amine gives N2 gas (aliphatic) or a diazonium salt (aromatic). A secondary amine gives a yellow oily nitrosamine. Tertiary amines give either a nitrite salt (aliphatic) or a nitroso ring compound (aromatic). But the sharpest single test that separates all three classes is the Hinsberg test.
No. Nitrous acid is unstable, so it is always prepared fresh in the flask by mixing sodium nitrite (NaNO2) with a cold mineral acid such as dilute HCl.
